Ok but think about it. If Muichiro became a hashira in 2 months. And he got at least affiliated with the demon slayer corps at 11.. then he would've become a hashira at 11-12
So you know how Yuichiro died at 11 to a demon right? Well this is an estimate but since he was taken in at that same age by demon slayers then we can infer that he became a slayer at 11-12